Ink brand permanency on metallic IC package lids has always been a critical issue in the semiconductor industry. Lid brands often identify information about the device type, the IC manufacturer, and the date of production. It is not uncommon for semiconductor users to subject IC packages to various chemical cleans before and after attachment to PC boards. If ink adhesion is not satisfactory and/or ink cure is incomplete, the brand may be degraded during the various cleaning operations and become partially or completely illegible. Long-term exposure to reactive ambients (e.g. high humidity, corrosive or polluted air, etc. ) can also cause slow degradation of poorly adhered or cured ink brands. Loss of the branding information can result in IC's which are unidentifiable. For this reason, MIL-STD tests have been developed and instituted in the semiconductor industry to evaluate ink brand permanency on package lids. Moisture resistance tests (and salt air exposure) can also help determine brand permanency under more aggressive conditions.
Various lid cleaning, inking, and curing methods have been developed and modified to enhance the brand permanency on metallic lids. One of the most difficult factors to control is the surface chemistry and cleanliness of the metal lid. This directly affects the bonding of the ink to the metal. The branding ink will not bond satisfactorily to lid surfaces that contain inordinate quantities of metallic oxides or hydrocarbons or other contaminants. These surface contaminants can accumulate from the thermal treatments during the lid seal process and burn-in. Normal workplace air exposure, as well as machine and human handling (e.g. fingerprints), can also contribute to surface contamination of the lid. Overall, the most common cause of brand loss is due to lifting or delaminating ink caused by poor ink bonding to the lid.
Current branding operations for metallic lids, and more specifically nickel plated lids such as electroplated or electroless plated nickel lids or gold plated nickel, involve the use of wet chemical cleans. These include basic solutions, which have pH values greater than 10 (e.g. Markem 535), and solvents like isopropyl alcohol (IPA), freon, and HCFC. These solutions are recommended as standard surface cleaning methods by the ink vendors and generally result in improved brand permanency compared to uncleaned lids. However, one disadvantage of these wet chemical cleans is the entire package must be immersed. The Markem 535 solution, for example, contains monoethanolamine and is quite basic (pH&gt;11). Hence, it tends to attack or etch various other materials associated with the packages. The attack is especially notable on the soldered leads, the lid-to-seal ring weld area, and the various plated metallic codes on the packages. This particular brand pre-clean can cause a darkening of these surfaces due to an accelerated build-up of oxides and the formation of `phospho-oxides` on electroless Ni plated surfaces. The ceramic package material is also etched. In most cases, these anomalies are cosmetic. However, many incidents of darkened leads and/or darkened weld areas are incorrectly assumed to be caused by latent corrosion or thermal stress, when in fact, they are caused by the brand pre-clean solution.
Lead solderability is also affected by the pre-branding cleans. The oxides and `phospho-oxides` are difficult to remove and can prevent good eutectic formation between the base metal and the solder. Leads solder dipped prior to brand also often exhibit a darkening and pitting of the solder. Reduced solderability is observed in these units as well.
Another drawback of the wet cleaning method is the use of solutions which are costly, difficult to dispose of, and are detrimental to the environment. Maintaining and changing these chemical baths, as well as keeping accurate records of bath changes and chemical disposal, are tedious and time consuming. In addition, several of these solutions can be potentially harmful to workers over a period of extended exposure.